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The Greek Model 440 BC: Leucippus & Democritus hypothesize that matter is made of atoms 440 BC: Leucippus & Democritus hypothesize that matter is made of atoms Explained why different substances have different properties Explained why different substances have different properties Performed no experiments Performed no experiments Very little evidence to support hypothesis Very little evidence to support hypothesis Democritus of Abdera

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The Greek Model Five major points Five major points 1. All matter is composed of atoms too small to be seen too small to be seen CANNOT be split into smaller parts CANNOT be split into smaller parts 2. There is a void, which is empty space between atoms 3. Atoms are completely solid 4. Atoms are homogeneous, with no internal structure 5. Atoms are different in size, shape, and weight

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The Dalton Model 1803: John Dalton (England) re-introduced the atomic idea to modern chemistry 1803: John Dalton (England) re-introduced the atomic idea to modern chemistry Dalton and other chemists did experiments involving gases and how they combine Dalton and other chemists did experiments involving gases and how they combine They noticed that elements always seemed to combine in the same ratios ( such as 1:1, 1:2, 2:3 etc) They noticed that elements always seemed to combine in the same ratios ( such as 1:1, 1:2, 2:3 etc)

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Dalton’s Atom, 1807 To explain these observations, Dalton hypothesized that: To explain these observations, Dalton hypothesized that: Chemical elements are made of atoms Chemical elements are made of atoms The atoms of the same element have the same mass The atoms of the same element have the same mass Atoms of different elements have different masses (but the same general shape) Atoms of different elements have different masses (but the same general shape) Atoms are involved in chemical reactions but are not changed by them Characteristics of Dalton’s atom uniformly dense indivisible solid spheres

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The Thomson Model 1897: J.J. Thomson hypothesized that there are areas of negative charge (electrons) within the atom 1897: J.J. Thomson hypothesized that there are areas of negative charge (electrons) within the atom Based on experimental observations Based on experimental observations Called the Plum Pudding Model Called the Plum Pudding Model

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Discovery of the Electron http://www.youtube.com/watch?v=IdTxGJjA4Jw

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Rutherford’s Gold Foil Experiment Rutherford shot alpha particles at gold foil expected the alpha particles to go through the gold foil but be deflected- most particles did Some alpha particles weren’t deflected, they bounced back! He said “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

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The Rutherford Model, 1911 Atomic theory was revised to explain Rutherford’s observations Atomic theory was revised to explain Rutherford’s observations Rutherford’s Model stated: Rutherford’s Model stated: The atom contains a tiny dense center called the nucleus The atom contains a tiny dense center called the nucleus the volume of the nucleus is about 1/10 trillionth the volume of the atom the volume of the nucleus is about 1/10 trillionth the volume of the atom The nucleus is essentially the entire mass of the atom (the rest is mostly empty space) The nucleus is essentially the entire mass of the atom (the rest is mostly empty space)

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The Rutherford Model, 1911 Rutherford’s Model stated: Rutherford’s Model stated: The nucleus is positively charged The nucleus is positively charged The amount of positive charge of the nucleus balances the negative charge of the electrons The amount of positive charge of the nucleus balances the negative charge of the electrons The electrons move around in The electrons move around in the empty space of the atom the empty space of the atom surrounding the nucleus surrounding the nucleus 3p+ e-

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Discovery of the Nucleus http://www.youtube.com/watch?v=wzALbzTdnc8

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The Bohr Model Niels Bohr Niels Bohr applies quantum theory to Rutherford's atomic structure applies quantum theory to Rutherford's atomic structure led to the idea of electrons moving around the nucleus in orbits or energy levels led to the idea of electrons moving around the nucleus in orbits or energy levels emission of light occurs when an electron moves into a lower energy orbit. emission of light occurs when an electron moves into a lower energy orbit.

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Bohr’s Research Electron orbit around the nucleus in seven different energy levels, or shells. Electron orbit around the nucleus in seven different energy levels, or shells. Electrons would only occupy the lowest possible energy level. Electrons would only occupy the lowest possible energy level. Electrons would only move up a level (increasing energy) if the lower levels were full. Electrons would only move up a level (increasing energy) if the lower levels were full.

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Modern Atomic Theory An updated version of Bohr’s model An updated version of Bohr’s model Electrons move in 3-D clouds rather than orbits Electrons move in 3-D clouds rather than orbits There are three There are three types of particles in the atom rather than two